1. Field of the Invention
The invention relates to an optical distance sensor according to the confocal optical imaging principle for the determination of height values and for three-dimensional surface measurement. Preferred fields of application are, especially, the inspection of soldered joints and the monitoring of the component quality in the case of high-density electronic printed circuit boards, such as for example multi-chip modules.
2. Description of the Related Art
In the inspection of the soldered joints or components of a printed circuit board, an investigation is essentially carried out for conductor track constrictions, impurities due to particles, soldered joint quality, correct placing of connecting pads, short circuits etc. As a result of the increasing intensification of the packing density of components in microelectronics, the inspection of three-dimensional objects with high resolution and at high speeds of inspection is becoming necessary. In particular, unequipped and equipped micromotherboards are to be automatically inspected.
Previously known devices for the recording of height raster images, which comprise a multiplicity of three-dimensionally existing points of object surfaces, are essentially based on the so-called triangulation method. In this method, a laser beam scans the surface of the object. The two planar space coordinates of a determined surface point are known by the relative position between scanning beam or illumination beam and the printed circuit board. The height coordinate of the surface point which is currently being measured is acquired by at least one laterally disposed objective in conjunction with a position-sensitive detector. In this way, it is possible to determine the three-dimensional space coordinates of a multiplicity of surface points. By the comparison of a recorded surface image with an ideal surface image and with consideration being given to specified error criteria, defects on printed circuit boards can be automatically recognized.
The abovementioned triangulation method has been further developed in various respects, but exhibits particular fundamental disadvantages:
there is the danger of secondary light influences if the detector receives the reflected light from surface points of the object which do not correspond to the current point of incidence. In the case of very glossy surfaces, this may lead to considerable errors of measurement.
Small objects which are situated very close to relatively large objects or in depressions cannot in every case be sensed, as a consequence of shadows.
In order to observe the Scheimpflug condition, in most cases a non-enlarging imaging onto the detector is required. In the case of small measurement spot sizes, this leads to high power densities. A high power density on the detector surfaces in the case of lateral photodiodes places an upper limit upon the speed of scanning. The use of photodiode arrays does not increase the data rate.
It is hitherto not known to vary the imaging scale and thus the resolving power by simple exchange, for example, of an objective on the sensor system.
Commercially available measuring systems according to the triangulation method exhibit particular refinements by means of which the abovementioned disadvantages are in part avoidable. Thus, the company Robotic Vision Systems (536 Broadhollow Road, Melville, N.Y. 11747, USA) has employed linear photodiode arrays in place of lateral photodiodes for the purpose of avoiding erroneous measurements due to secondary reflexes. Erroneous measurements are recognized and eliminated by appropriate evaluating software. In total, however, the data rate of the system is reduced so greatly that this system is not suitable for a full inspection in a processing line.
In order to cope with the problems in the application of a lateral photodiode, the company Matsushita Kotobuki Electric Co., Ltd. (2131 Ohara-minamikata, Kawauchi-machi, Onsen-gun, Ekimeken 791-03, Japan) has developed a system with which observation takes place from eight directions. An evaluation of the detector signals is executed by appropriate algorithms. The reliability of recognition in the case of glossy surfaces is enhanced as a result of this. The overall construction becomes very costly as a result of the use of approximately four sensor heads each having eight detectors, which scan by means of a rapidly rotating disk. Over and above this, the system is unable to measure within deep holes. A resolution greater than 40 .mu.m is not achieved, on account of mechanical and optical adjustment problems in the course of the rapid rotation of the sensor head.
The company Nagoya Electric Works Co., Ltd (550 Takawari, Katori, Tadocho, Kuwana-gun, Mieken 511-01, Japan) scans with a laser beam over an equipped circuit board and measures the angle of the specular reflection. By the evaluation of the surface inclination of soldered joints or components, the absolute height can be determined by integration. Inclinations which amount to more than 45.degree. cannot however be detected. As a result of this, the height of objects with vertical walls cannot be measured. A measurement within small holes is not possible and secondary reflexes or reflections cannot be eliminated.
A likewise known system is offered by the company Omron Institute of Life Science (17 Chudoji minami-cho, Shimogyo-ku, Kyoto 600, Japan). In this case, the height information is obtained from the specularly reflected light. The specimen is illuminated from various directions with three different colors. The specularly reflected light is detected by a color camera and the inclination of the specimen surface is computed. The power level approximately corresponds to the system from Nagoya.
In general, it can be stated that secondary reflexes or reflections occurring on neighboring soldered joints, especially in the case of soldered joints with specular surfaces, result in erroneous information and correspondingly false height values if detector surfaces are large. The use of small detector surfaces is to be aimed at, since as a result of this only the immediate environment of the measurement location to be instantaneously imaged is acquired. This is taken into consideration by the use of a synchronized triangulation scanner. In this case, the detection beams are directed by means of two laterally fitted deflecting devices via the scanning objective and the beam deflection unit (a rotating polygonal mirror) onto the detector surface. As a result of the synchronous beam deflection of the illumination beam and measuring beam (detection beam), only the height movement of the point of incidence is imaged onto the detector, whereby the latter can be dimensioned to be correspondingly narrow and secondary reflexes in the direction of scanning are masked out. Disturbing reflexes occurring perpendicular to the direction of scanning cannot be illuminated thereby. As a result of the design of the system, the point of incidence of the light or the measurement location can be observed only from two directions in space. In the case of high-density circuit boards, this leads to considerable occlusions.
A method which has already proved to be useful in the three-dimensional measurement of structures is based on the confocal principle. In this case, a point light source which is usually defined by an aperture diaphragm is imaged onto the specimen or the object. The backscattered light is again imaged onto an almost punctiform detector. In this case, the maximum light intensity impinges only on the detector (photodetector) if the object plane and the detector plane actually lie at the focus of the respective optical system (confocally). If the object is situated outside the focal plane, then the result is a great broadening of the measurement beam ahead of the point detector, whereby the measurable intensity decreases to a great extent.
A sensor based on the confocal principle is described, by way of example, in the article-- 3-D profile detection of etched patterns using a laser scanner; Moritoshi Ando et al; Proceedings of SPIE, Vol 389, Optical Systems Engineering III; Los Angeles, Calif., USA; 20th-21st Jan. 1989. In particular in FIGS. 2 and 3, it is shown that the object plane and the sensor plane lie in each instance in the region of the focus. Furthermore, this article describes the use of scanning lenses, as well as a rotating polygonal mirror as beam deflection unit. According to the confocal principle, the axes of illumination beam and detection beam are identical in the region of the objective.